In a simple mass spectrometry (MS) system, ions of a sample are formed in an ion source, such as for instance an Electron Impact (EI) source, an electrospray (ESI) source or an Atmospheric Pressure Ionization (API) source. The ions then pass through a mass analyzer, such as for instance a quadrupole (Q) or a time of flight (TOF) device, for detection. The detected ions include at least one of molecular ions, fragments of the molecular ions, and fragments of other fragment ions.
Tandem mass spectrometry (MS/MS) systems have been developed and are well known. Such tandem systems are characterized by having two or more sequential stages of mass analysis and an intermediate ion fragmentation, where ions from the first stage are fragmented into product ions for analysis within the second stage. There are two basic types of tandem mass spectrometers, namely those that are “tandem in space” and those that are “tandem in time.” Tandem-in-space mass spectrometers, such as for instance triple quadrupole (QqQ) and quadrupole-time of flight (Q-TOF) devices, have two distinct mass analyzers, one for precursor ion selection and one for product ion detection and/or measurement. An ion fragmentation device, such as for instance a gas-filled collision cell, is disposed between the two mass analyzers for receiving ions from the first mass analyzer and for fragmenting the ions to form product ions for introduction into the second mass analyzer. Tandem-in-time instruments, on the other hand, have one mass analyzer that analyses both the precursor ions and the product ions, but that does so sequentially in time. Ion trap and FT-ICR are two common types of mass spectrometer that are used for tandem in time MS/MS.
Several MS/MS scan types, in particular “product ion scan”, “precursor ion scan” “neutral loss scan,” and Selected Reaction Monitoring (SRM) scan are known. Performing a “product ion scan” is done by selecting a particular precursor ion in the first MS stage, and then obtaining in the second MS stage a full scan of the product ions that are formed when the selected precursor ion is fragmented. A “precursor scan,” is a method that has a fixed product ion selection for the second MS stage, while using the first MS stage to scan all of the pre-fragmentation precursor ions in a sample. Detection is limited to only those molecules/compounds in the sample that produce a specific product ion when fragmented. In the SRM mode, only a specific precursor/product ion pair is monitored. Multiple precursor/product ion pairs can be monitored during a specific analysis. Finally, a “neutral loss scan” is a method that supports detection of all precursor ions that lose a particular mass (non-charged) during fragmentation. The second stage mass analyzer scans the ions together with the first stage mass analyzer, but with a predetermined offset corresponding to the lost mass. Neutral loss scans are used for screening experiments, where a group of compounds all give the same mass loss during fragmentation.
In theory and in practice, the steps of selecting ions and fragmenting the selected ions can be repeated iteratively. For instance, an MS/MS/MS (or MS3) analysis would include a precursor ion selection step, a fragmentation step that produces first-generation product ions by fragmentation of the selected precursor ion(s), a product-ion selection step, a second fragmentation step that produces second-generation product ions from the selected first-generation product ions and, finally, mass analysis of the second-generation product ions. The symbolism MSN (N an integer) is sometimes used to indicate tandem mass spectrometry experiments that include N generations of ions (a first generation consisting of precursor ions followed by N−1 generations of product ions). According to this same scheme, simple, non-tandem mass spectrometry is denoted by MS1 or, simply, MS.
FIG. 1A is a schematic illustration of an example of a conventional mass spectrometer system, shown generally at 200, capable of providing collisional ion dissociation. Referring to FIG. 1A, an ion source 212 housed in an ionization chamber 24 is connected to receive a liquid or gaseous sample from an associated apparatus such as for instance a liquid chromatograph or syringe pump through a capillary 207. As but one example, an atmospheric pressure electrospray source is illustrated. However, any ion source may be employed, such as a heated electrospray ionization (H-ESI) source, an atmospheric pressure chemical ionization (APCI) source, an atmospheric pressure matrix assisted laser desorption (MALDI) source, a photoionization source, or a source employing any other ionization technique or a combination of the above techniques. The ion source 212 forms charged particles 209 (either ions or charged droplets that may be desolvated so as to release ions) representative of the sample. The charged particles 209 are subsequently transported from the ion source 212 to the mass analyzer 39 in high-vacuum chamber 226 through intermediate-vacuum chambers 218 and 225 of successively lower pressure in the direction of ion travel. In particular, the droplets or ions are entrained in a background gas and may be transported from the ion source 212 through an ion transfer tube 216 that passes through a first partition element or wall 215a into an intermediate-vacuum chamber 218 which is maintained at a lower pressure than the pressure of the ionization chamber 24 but at a higher pressure than the pressure of the high-vacuum chamber 226. The ion transfer tube 216 may be physically coupled to a heating element or block 223 that provides heat to the gas and entrained particles in the ion transfer tube so as to aid in desolvation of charged droplets so as to thereby release free ions.
Due to the differences in pressure between the ionization chamber 24 and the intermediate-vacuum chamber 218 (FIG. 1A), gases and entrained ions are caused to flow through ion transfer tube 216 into the intermediate-vacuum chamber 218. A second plate or partition element or wall 215b separates the intermediate-vacuum chamber 218 from a second intermediate-pressure region 225, likewise a third plate or partition element or wall 215c separates the second intermediate pressure region 225 from the high-vacuum chamber 226. A first ion optical assembly 27a provides an electric field that guides and focuses the ion stream leaving ion transfer tube 216 through an aperture 222 in the second partition element or wall 215b that may be an aperture of a skimmer 221. A second ion optical assembly 27b may be provided so as to transfer or guide ions to an aperture 227 in the third plate or partition element or wall 215c and, similarly, another ion optical assembly 27c may be provided in the high vacuum chamber 226 containing a mass analyzer 39. The ion optical assemblies or lenses 27a-27c may comprise transfer elements, such as, for instance a multipole ion guide, so as to direct the ions through aperture 222 and into the mass analyzer 39. The mass analyzer 39 comprises one or more detectors 48 whose output can be displayed as a mass spectrum. Vacuum ports 213, 217 and 219 may be used for evacuation of the various vacuum chambers.
The mass spectrometer system 200 (as well as other such systems illustrated herein) is in electronic communication with a controller 15 which includes hardware and/or software logic for performing data analysis and control functions. Such controller may be implemented in any suitable form, such as one or a combination of specialized or general purpose processors, field-programmable gate arrays, and application-specific circuitry. In operation, the controller effects desired functions of the mass spectrometer system (e.g., analytical scans, isolation, and dissociation) by adjusting voltages (for instance, RF, DC and AC voltages) applied to the various electrodes of ion optical assemblies 27a-27c and quadrupoles or mass analyzers 33, 36 and 39, and also receives and processes signals from detectors 48. The controller 15 may be additionally configured to store and run data-dependent methods in which output actions are selected and executed in real time based on the application of input criteria to the acquired mass spectral data. The data-dependent methods, as well as the other control and data analysis functions, will typically be encoded in software or firmware instructions executed by controller. A power source 18 supplies an RF voltage to electrodes of the devices and a voltage source 21 is configured to supply DC voltages to predetermined devices.
As illustrated in FIG. 1A, the conventional ion trap mass spectrometer system 200 is a triple-quadrupole system comprising a first quadrupole device 33, a second quadrupole device 36 and a third quadrupole device 39, the last of which is a mass analyzer comprising one or more ion detectors 48. The first, second and third quadrupole devices may be denoted as, using common terminology, as Q1, Q2 and Q3, respectively. A lens stack 34 disposed at the ion entrance to the second quadrupole device 36 may be used to provide a first voltage point along the ions' path. The lens stack 34 may be used in conjunction with ion optical elements along the path after stack 34 to impart additional kinetic energy to the ions. The additional kinetic energy is utilized in order to effect collisions between ions and neutral gas molecules within the second quadrupole device 36. If collisions are desired, the voltage of all ion optical elements (not shown) after lens stack 34 are lowered relative to lens stack 34 so as to provide a potential energy difference which imparts the necessary kinetic energy.
Various modes of operation of the triple quadrupole system 200 are known. In some modes of operation, the first quadrupole device is operated as an ion trap which is capable of retaining and isolating selected precursor ions (that is, ions of a certain mass-to-charge ratio, m/z) which are then transported to the second quadrupole device 36. More commonly, the first quadrupole device may be operated as a mass filter such that only ions having a certain restricted range of mass-to-charge ratios are transmitted therethrough while ions having other mass-to-charge ratios are ejected away from the ion path 45. In many modes of operation, the second quadrupole device is employed as a fragmentation device or collision cell which causes collision induced fragmentation of selected precursor ions through interaction with molecules of an inert collision gas introduced through tube 235 into a collision cell chamber 37. The second quadrupole 36 may be operated as an RF-only device which functions as an ion transmission device for a broad range of mass-to-charge ratios. In an alternative mode of operation, the second quadrupole may be operated as a second ion trap. The precursor and/or fragment ions are transmitted from the second quadrupole device 36 to the third quadrupole device 39 for mass analysis of the various ions.
The ion optical assemblies 27a-27c and quadrupole devices 33, 36, 39, as known to those of ordinary skill in the art, can form an ion path 45 from the ionization chamber 24 to at least one detector 48. The electronic controller 15 is operably coupled to the various devices including pumps, sensors, ion source, ion guides, collision cells and detectors to control the devices and conditions at the various locations throughout the mass spectrometer system 200, as well as to receive and send signals representing the particles being analyzed. If the second quadrupole device 36 is to be used only as a collision or fragmentation cell (or, in general, a reaction cell), then the second quadrupole device may be replaced by a hexapole or higher order multipole device or any other device that acts similarly, such as a stacked ring ion guide.
FIG. 1B illustrates a portion of a mass spectrometer system including a curved collision cell 36. Other not-illustrated components of the mass spectrometer system may be similar to those illustrated in FIG. 1A. Because of the curved shape of the collision cell 36, which is also denoted as Q2, the first 33 and third 39 quadrupole devices (also denoted Q1 and Q3, respectively) are oriented at an angle to one another and the ion path 45 is correspondingly curved. Ions are maintained within the collision cell 36 in the usual fashion by the confining effects of the quadrupole fields generated by oscillating potentials applied to the curved rod electrodes comprising the collision cell 36. In addition, auxiliary electrodes may be disposed within or around the collision cell in order to provide a drag field within the collision cell that functions to urge ions through the collision cell along the curved ion path 45. The configuration of quadrupole devices shown in FIG. 1A aids in manufacturing a compact size mass spectrometer and also facilitates separation of ions from neutral gas molecules, which are not deflected along the curved portion of the ion path 45.
Quadrupole scanning mass spectrometers operate by RF and DC voltages applied to various electrodes over time. Calibrations are used to convert voltage values into m/z values and to convert detected intensity values into abundance values. A full tuning and calibration procedure includes adjustments and optimizations of an ion source, lenses and detectors followed by introduction into the instrument of one or more calibrant compounds that yield ions having well-known m/z and intensity values. Such tuning and calibration procedures may be performed at regular intervals—for instance, weekly or monthly. Unfortunately, however, instrumental operation can drift with time between regularly scheduled calibrations, diminishing the accuracy of prior calibrations and requiring more frequent monitoring and calibration.
In high throughput clinical laboratory settings, it is important that instrument calibrations remain up-to-date. However, in these same environments, it is often inconvenient to perform frequent un-scheduled re-calibrations, since numerous urgent analyses of patient samples may be delayed. Accordingly, there is a need for methods and apparatus that can perform quick calibration tests and minor calibration adjustments to compensate for instrumental drift without requiring a full instrumental tuning and re-calibration procedure.